This invention relates generally to ferroelectric memory cells. More particularly, the present invention relates to an iridium oxide local interconnect method and a corresponding ferroelectric memory cell.
A one-transistor, one-capacitor ("1T-1C") ferroelectric memory cell 10 is shown in schematic form in FIG. 1. Memory cell 10 is a three-terminal device having a bit line node 12 and a word line node 14, in similar fashion to a DRAM cell, but in addition, having a plate line node 16 for poling a ferroelectric capacitor 20. Memory cell 10 includes an MOS transistor 18, the gate forming the word line node 14, and a source/drain forming the bit line node 12. One electrode of ferroelectric capacitor 20 is coupled to the other source/drain of transistor 18 at intermediate node 19, and the other electrode of ferroelectric capacitor 20 forms the plate line node 16. Memory cell 10 is typically included in an array of such cells, not shown in FIG. 1, arranged in rows and columns.
In the prior art, a "local interconnect" or short metal strap is used to physically form the intermediate node 19 in an integrated circuit implementation of memory cell 10. A common material used in such a local interconnect is titanium nitride ("TiN") because of its low resistivity and ability to interface with both silicon and platinum. One problem with titanium nitride is that this material can cause water molecules found in oxide layers to dissociate into hydrogen and oxygen ions. Oxide layers are commonly used in the fabrication of integrated circuit ferroelectric memory cells, therefore hydrogen can come into contact with the dielectric layer of the ferroelectric capacitor 20 as a result of water or hydroxyl ion dissociation. It is well known that hydrogen can cause the electrical performance of ferroelectric materials to degrade, by reducing or entirely eliminating the switched charge necessary for discerning a valid data state. The production of hydrogen and consequent degradation undesirably increases at elevated annealing or packaging temperatures above 400.degree. C.
Iridium oxide is known as an electrode material in the prior art, but it does not form a reliable ohmic contact directly with silicon. Iridium oxide is therefore not recommended as the sole local interconnect material.
What is desired, therefore, is a method for forming a low resistivity local interconnect for a ferroelectric memory cell that will not degrade the performance of the ferroelectric dielectric layer, even at elevated temperatures above 400.degree. C.